The low sound or low frequency was heretofore not deemed to be important in the sound reproduction of the TV receiver. With the TV receiver in recent years, however, it has been intended to create high quality sounds by emphasizing the range of low sounds, especially heavy low sounds, thereby attempting to simulate the perception at a live performance. For example, there has been developed 3D (3-dimensions) system which R and L signals in the middle and high frequency ranges during stereophonic broadcasting are reproduced through the right and left speakers for middle and high sounds, and the signals in the low frequency range are reproduced through a single speaker for low sounds.
The low frequency range generally includes the sounds having frequency in the range of 20 to 200 Hz. However, the heavy low frequency range does not have a distinct definition on its range. For example, the sounds having the frequency of up to 100 Hz, of up to 70 Hz or of up to 50 Hz or the like are generally deemed as being included in the heavy low frequency range.
Japanese Unexamined Utility Model Publication SHO.-58-54177 discloses a TV receiver having installed therein a speaker system adapted for reproduction of the low sound. FIG. 9 shows the TV cabinet disclosed in this publication. With this TV cabinet, a cabinet 6 has a lower section having installed therein a speaker box 64 in which a heavy low sound speaker 4 and a drone cone 5 are mounted.
Further, Japanese Unexamined Utility Model Publication SHO.58-50781 discloses a speaker system provided at the lower portion of the television cabinet 6, as shown in FIG. 10. The speaker system comprises the driving speakers 63, 63 for middle and high sounds (mid-frequency and high frequency speaker drivers), and the driving speakers 4, 4 for low sounds (low frequency speaker driver) having connected thereto horns 7, 7.
With the foregoing speaker systems, however, it is required for reproduction of the low sounds to expand a volume of the speaker box or the horn. Therefore, there was a drawback that the entire volume of the TV receiver must be enlarged.
The speaker having a voice coil is provided with a speaker box or a horn so as to boost sound pressure for reproducing the low sound. This implementation is based on a kind of the columnar resonance principle. For example, with reference to FIG. 7a, a cylindrical horn 65 has one end having mounted therewith a driving speaker 4 and other end having opening. When the sound signal is applied to the driving speaker 4 and the frequency of the sound signal is varied, the microphone 66 positioned at the place away from the speaker 4 by distance "d" revealed ripple characteristics in the waveform of frequency characteristics, as shown in FIG. 7b.
The resonance frequency is given by the formula as follows: ##EQU1## where n is a positive integer, l is a length of the cylindrical horn and C is a sound velocity.
The sound pressure received by the microphone 66 had such characteristics that the peaks of the sound pressure were generated at almost the same positions as those obtained by calculation of the resonance frequency of the cylindrical horn. This proves that the actual sound pressure characteristic agrees well with the theoretical results. Such a cylindrical horn is not suited for use in the speaker system for low sounds since the sound pressure varies depending upon the frequency of the sound signal.
With reference top FIG. 8a, the cylindrical horn is formed in U-shape and has other end opening toward the front. In case of this cylindrical horn, when sound signals are fed to the driving speaker 4, the sound is transmitted from both the speaker 4 and the horn opening 14 to the microphone 66 having distance "d" away from the horn. The sounds from the speaker 4 and the horn opening 14 are increased or boosted with each other when the phases of the sound are brought into coincidence at the microphone 14, but are reduced when the phases of the sound shift from each other by 180.degree.. Therefore, the resultant waveform characteristics become complicated due to interference, as shown in FIG. 8 with a broken line.
The inventors have found that when the drone cone 5 is attached to the horn opening 14 the drone cone 5 is caused to vibrate in accordance with the pressure change within the horn 65 generated by the vibration of the speaker driver 4, resulting in the frequency characteristics as shown in FIG. 8b with a solid line. Since the drone cone 5 has a vibrating plate or diaphragm but no driving coil, the diaphragm vibrates only by the vibration of air produced in the horn 65. This speaker system has a higher sound pressure in the range of 30-100 Hz, in comparison with that of the speaker system having no drone cone. Further, it has been found also in the range of 200-600 Hz that the speaker system with drone cone has more improved characteristics than the system having no drone cone.
The speaker system of the present invention has been accomplished by the foregoing recognition.